2019-2020 electrical capacitance of cotton roots

We conducted two rounds of plastic pot experiments (in 2019 and 2020), one PVC tube experiment (in 2020) and one field experiment (in 2020) at the Luancheng Agro-Eco-Experimental Station of the Chinese Academy of Sciences (37°53′N, 114°41′E, altitude: 50.1 m) located in Shijiazhuang, China.The ECroot and soil electrical capacitance (ECsoil) were measured with a Passive Component LCR Meter (EXTECH LCR200, USA) in a parallel equivalent circuit at 100 Hz, 1 kHz, and 10 kHz AC frequencies with 1 V terminal voltage. For Pot + Field topsoil experiments, we measured the ECroot before and after severing the root with a sharp knife (the knife was inserted at an angle of approximately 45 degrees from the horizontal plane and approximately 5 cm from the stem.). Except for the Tube + Mixed topsoil experiment, all the ECroot and ECsoil measurements followed similar steps as the other three experiments. When measuring ECroot, the negative electrode was inserted into the soil to 12 cm through a stainless steel rod (13 cm long and 3 mm in diameter), which was positioned 10 cm away from the stem base. The positive electrode (round rod stainless steel and 1 mm in diameter) was inserted into the center of the residual plant stem at 1 cm above the soil surface. Two stainless steel electrodes with a diameter of 3 mm and a distance of 20 cm were inserted into the soil to a depth of 12 cm to measure ECsoil.For the Tube + Mixed topsoil experiment, the same positive electrodes were used as in the other three experiments. The negative electrode was inserted into the soil at the side of the pipe wall and 10 cm from the top end of the PVC pipe. After the measurement was completed, the plant electrode was fixed, and the ground electrode was sequentially moved 10 cm downward for the next measurement. Each PVC pipe was measured 6 times down to 60 cm. We pretested the effects of varying the distance of the soil electrode (at the same soil layer) on the capacitance measurements.All the roots were separated from the soil by rinsing through a stainless steel sieve (0.5 mm mesh). We carefully washed soil from the roots and retained most of the root material in the pot and PVC tube experiments. Root samples were stored in a freezer (-20 °C) until they could be washed and separated. The debris was manually removed from washed root samples. The cleaned roots were scanned on a flatbed scanner (Epson Expression 12000XL, USA), and root length, surface area and volume were analyzed with WinRhizo software (ver. 2019a, Reagent Instruments Inc., Quebec, Canada). In the Pot + Field topsoil experiment, only the tap root and the attached lateral roots were scanned. Some roots broken during cleaning were not scanned. In the Pot + Mixed topsoil, Tube + Mixed topsoil and Field experiments, all roots were scanned. All roots were subsequently oven-dried to a constant mass (75 °C for 72 h), and the dry weight was recorded.At the time of root harvest, a subsample of soil was collected, and the soil water content was measured by the oven-drying method (drying for 24 h at 105 °C).